Claire just got a mobile internet bill for €7,500. To say the young Irish woman was shocked is an understatement. She had been one week in the US.

Her experience is a big warning to mobile users. She is bright, competent and communications-savvy. She would not willingly incur such a whopping bill.

Planned reductions in EU roaming charges from 2015 will not change the fact that outside the EU, or when your phone is stolen, users can quickly become liable for massive bills.

Claire is not her real name. She is a rising manager who would be mortified if people thought that she had incurred such costs recklessly. So she wants to remain anonymous.

She went to America for a relative’s birthday. Her problems started when she turned off the €50 ceiling that mobile phone companies automatically apply to their customers

roaming abroad. But Claire is adamant that she turned this off only to view her news feed on Facebook a few times and to try to use a Google map that did not download.

She was previously stung for hundreds of euro for internet access while in France and learnt from that experience. She says that she carefully turned off roaming on this occasion when she was not using it.

But the local US mobile company to which her phone connected has billed 02 for 960 megabytes of use by her, at about €8 per megabyte. This compares to the standard rate of 55 cent per megabyte within the EU.

The usage for which Claire has been billed, and which she denies, amounts to more in a week than what the average Irish mobile customer uses in a month (717 megabytes). Claire was staying most of the time where there was wi-fi, which she used instead of roaming.

This is a case of ‘bill shock’ as the Americans now call it, and she is far from the only person to have experienced it on their return from abroad.

Just last month, the Citizens’ Advice organisation in Britain called for mobile phone providers to do more to counter ‘bill shock’. The charity reported that 130,000 consumers contacted it in 2013 for

advice about unexpectedly high bills, which are driving victims into serious debt.

The moral of Claire’s story is that you should turn off roaming and leave it off, especially when you exit the EU.

It does help to know where you are. uSwitch, which describes itself as “a free, impartial online and telephone comparison and switching service” in the UK, points out that almost half of all Britons “incorrectly believe Croatia is in the EU, while 30 per cent think Turkey is — leaving them vulnerable to high mobile roaming charges”.

And remember that if you do not turn off roaming and if your mobile company blocks your phone after using 50 megabytes of data, then if you text them to remove that block, you will not be warned again.

It is almost impossible to guess precisely how many megabytes you will use when roaming is turned on. It is not a matter of minutes, like a mobile telephone call. It is a combination of factors including speed and the size of any images and other factors.

O2 says that one factor that can add to the bill is any automatic download or update that occurs when roaming is on, even where you are not directly using the internet.

Connecting the phone as a modem to a laptop can also escalate costs considerably. One agent claimed that some phones may not always turn off roaming effectively.

O2 says that data usage continues to increase as a result of network improvements and greater adoption of data-intensive mobile applications. And it seems that a surprising number of people take risks.

uSwitch has found that “almost six in 10 Brits won’t check charges for using their phones abroad before they jet off this year, but more than four in 10 plan to use their phones frequently”.

The thisismoney.co.uk website reports the case of one young English Facebook user who went to Egypt for a fortnight and took her iPhone. She regularly updated the social networking site. “But to her horror, when she returned home her provider, T-Mobile, landed her with a £4,000 (€4.900) bill — quadruple the cost of a package holiday”.

Mobile phone operators could do more to warn people. The €50 block is a good idea, but it needs to be repeated after each usage of €50 and not just applied once. Small print in contracts, or warnings on wordy webpages, are not enough.

Phone companies do sometimes intervene to block total usage when it is clearly excessive. But by the time 02 did this in Claire's case, while she was still in America, she had already clocked up her bill of over €7,000.

Your bill will never show you afterwards what websites you have used or what the operator claims you have used. You will just be told how many megabytes you apparently used at particular times.

One 02 agent told me that a single download of a US series like Grey’s Anatomy could cost you between €6,000 and €10,000, depending on the local US mobile phone company and the quality of picture.

Asked if O2 has to pay to the US company the actual amount billed to its own customers for their American mobile internet use, or if there is a massive wholesale bulk discount for O2, the Irish provider declined to comment.

In Claire’s case, O2 presented her with a bill for €7,500, which it subsequently reduced to €1,500 after representations and enquiries were made about the bill. Claire continued to protest and to insist that she had used internet access carefully. O2 pointed out that customers going to the US have an option to purchase in advance an extra gigabyte of usage for €110.70.

O2 now says that “as a gesture of good will” for a loyal customer, it will on this occasion treat Claire as if she had, in fact, purchased that extra capacity before she left and will thus reduce her final total bill for internet access in the US to €110.70.

She’s very relieved. But how many more people are in for a similar bill shock this year?